A vehicle suspension connects a vehicle to its wheels and in a manner that allows relative movement between the chassis of the vehicle and the wheels. The vehicle suspension facilitates accurate steering and handling of the vehicle on a variety of surfaces while promoting a comfortable ride for the passengers of the vehicle. Conventional steering systems have a direct mechanical connection of a steering wheel to the steered wheels. In some systems, this connection includes a power-assisted steering system. Examples of power-assisted steering systems include a hydraulic power-assisted steering (HPAS) system and an electrically power-assisted steering (EPAS) system.
Vehicle steering and suspension components are typically designed and configured to provide a mechanical alignment of the wheels. The mechanical alignment of the wheels enhances the experience of the driver, such as by compensation for wheel disturbances (e.g., wheel imbalance, shimmy effects, design tolerances, and uneven tire wear), providing directional stability, providing an enhanced steering feel for the driver, and causing self-alignment of the wheels (e.g., a tendency to align the wheels so that the vehicle may move forward instead of turning).
To provide the attributes mentioned above, the steering axis of each wheel to be used for steering is typically inclined with respect to the vertical axis of the wheel, both in side view as well as in front or back view. The steering axis is also typically offset with respect to the center of the wheel. Based on these steering axis properties, moment arms are generated in the ground with the respect to a contact patch (e.g., the area of contact between the tire and the ground). More specifically, a kingpin trail (e.g., the distance, viewed from the front or back of the vehicle, from the center of the contact patch to the intersection of the steering axis with the ground) provides a moment arm that generates an aligning moment about the steering axis due to longitudinal forces at the tire, such as acceleration and braking. A caster trail (e.g., the distance, viewed from the side of the vehicle, from the center of the contact patch to the intersection of the steering axis with the ground) facilitates a moment arm that generates an aligning moment about the steering axis due to lateral forces at the wheel, such as while cornering or turning. However, these moments also tend to produce “jacking” (e.g., lifting or diving) effects at the center of the wheel, causing the body of the vehicle to lift or dive in reaction thereto.
In addition, the tires typically employed as part of the wheels of a vehicle tend to provide steering self-alignment while cornering. More specifically, during a turn, lateral forces tend to be concentrated behind the center of the contact patch at a pneumatic trail. Due to those forces, a lateral force toward the direction of the turn is produced at the pneumatic trail, thus causing a self-aligning torque on the wheel.
Due to both the mechanical and pneumatic attributes discussed above, the HPAS or EPAS system employed to assist the driver in steering the vehicle must continually overcome the mechanical self-alignment, pneumatic self-alignment, and jacking effects described above to perform the steering indicated by the driver. Moreover, the inclined steering axis also tends to increase tire wear and wheel rolling resistance.